Voltage regulators have a wide area of use in electronics for the purpose of providing a supply voltage. Voltage regulators in general can be subdivided into two classes, switched voltage regulators and non-switched or linear voltage regulators. In comparison with a linear voltage regulator, switched voltage regulators have the particular advantage that the power loss does not depend on the input voltage. In contrast, linear voltage regulators have the particular advantage that the output voltage is particularly precise and stable. Linear voltage regulators should be able to attenuate interference which occurs at their input or at their output. On account of this, linear voltage regulators can be used wherever interference occurs at the input, for example downstream of a switched voltage regulator for the purpose of smoothing the voltage spikes in an electrical system of an automobile.
As a result of the increasing use of electronics, spikes in the supply voltage also occur more and more often in battery-supported systems and have to be attenuated by a voltage regulator. One example of this is the electrical system of an automobile. All applications in which digital technology is used are affected by this since switching operations induce voltage spikes in the supply voltage. Voltage spikes or interference spikes can occur at the input, the output or the ground connection of a voltage regulator.
Interference spikes at the input of the voltage regulator occur, for example, if the input voltage is provided by a switched voltage regulator. Interference spikes at the input also occur if the input voltage is provided by a battery-supported system, this input voltage being loaded by further connected loads.
Interference spikes at the output of the voltage regulator occur, for example, if digital technology or switches is/are used at the output. The interference spikes may also be caused by other sources.
The ability of a voltage regulator to withstand these interference spikes or transient influences at its connections is reflected in the data sheet by the parameters PSSR and “Input Voltage Transient Immunity”, where PSSR is the “Power Supply Rejection Ratio” which is a measure of the sensitivity of a circuit to influences of its supply voltage.
The ability of a voltage regulator to attenuate interference spikes can be improved by increasing the output capacitor. Such an output capacitor buffers the current provided by the voltage regulator, with the result that a connected load can draw the required current. An increased output capacitor has the disadvantages, inter alia, that both the costs and the space taken up on the printed circuit board increase. The regulating speed of the voltage regulator decreases.
The sensitivity of a voltage regulator to interference spikes can be improved by using an input capacitor or an input filter. Like in the case of an increased output capacitor, both the costs and the space taken up on the printed circuit board increase.
The sensitivity of a voltage regulator to interference spikes can be improved by increasing the bias current, that current of the voltage regulator which adjusts all relevant currents of the voltage regulator being referred to as the bias current. An increase in the bias current increases the regulating speed, the current draw and the quiescent current. An increase in the current draw is undesirable in most cases.
U.S. Pat. No. 6,541,946 shows a positive feedback circuit with a high-pass filter for improving the PSSR “Power Supply Rejection Ratio”.
IEEE Transaction on Circuits and Systems “Full On-Chip Low-Dropout Voltage Regulator” R. Milliken et al. shows a compensation circuit for improving the sensitivity of a voltage regulator to interference spikes.